Relationship Between Soil Nutrients Of Carbon And Nitrogen And Microbial Activity In Different Successional Stages Of Biological Crusts In Typical Hilly Areas Of The Loess Plateau

Biological soil crusts(BSCs)occur on or within the top soil surface(2mm-20mm)in arid and semi-arid areas.They are complex organic integrity consisted of eukaryotic microalgae,cyanobacteria,bacteria,fungi,lichen and moss.BSCs play critical ecological roles in accumulating nutrients,maintaining moisture,reducing wind erosion and water erosion,facilitating biogeochemical cycles,and enhancing vegetation succession.BSCs can be divided into algal crusts,lichen crusts and moss crusts according to dominant species composition.Algal crusts was the early stage of BSCs.With the development of algal crusts,lichen crusts and moss crusts are appeared and increased gradually in the later stage of succession.Microorganisms in BSCs mediate many important metabolic processes of carbon and nitrogen cycle and play an key role in soil carbon and nitrogen transformation.In the typical hilly areas of the Loess Plateau,we selected bare sand,algal crusts,lichen crusts and moss crusts in this study.To understand critical ecological function of BSCs in ecological restoration in Loess Plateau,we investigate the variation of soil physicochemical properties,microbial biomass and enzyme activities in different successional stages of BSCs.The relation of soil nutrients related C and N,microbial biomass and enzyme activities were also analyzed in the study.The results of study can provide important theoretical basis for the regional ecological restoration.The succession of BSCs is beneficial to the accumulation of the nutrients of soil carbon and nitrogen in the surface and subsoil,and then improve the fertility of soil.The content of soil organic carbon(SOC)showed a significant increasing trend with the succession process of BSCs(P<0.05),which was specifically presented as moss crusts>lichen crusts>algal crusts>bare sand.In the lower layer of the BSCs,the content of SOC was also significantly higher than the bare sand in different types of BSCs(P<0.05),but there was no significant difference between the three types of BSCs.The content of SOC in the BSCs layer was significantly higher than that in the lower layer(P<0.01),indicating that the content of SOC in the BSCs layer was significantly increased by BSCs.The contents of TN,NH₄⁺and NO₃^-is similar to that of organic carbon in terms of the law of evolution,that is,it shows a significant increasing trend with the succession BSCs(P<0.05).In the lower layer of BSCs,the contents of NH₄⁺and NO₃^-also increased significantly in lichen crusts and moss crusts.The contents of TN and NH₄⁺in the moss crust were significantly higher than those in the lower BSCs(P<0.01).The succession of BSCs can increase the biomass of algae and heterotrophic microorganisms.In the BSCs layer,the biomass of chlorophyll a and b,carotenoids,bacteria and fungi increased significantly with the succession of BSCs(P<0.05).There was no significant difference in the number of bacteria,fungi and actinomycetes in the lower layer.Microbial biomass carbon and nitrogen are active nutrient pools of soil.The soil microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN)were also significantly increased with the succession of BSCs(P<0.05).Texture type of soil had a certain effect on the microbial biomass of BSCs,but the difference was not significant(P<0.05).The succession of BSCs can increase the activities of enzymes related to carbon and nitrogen conversion in soil.In the BSCs layer,the activities of most carbonolytic enzymes,such as sucrase,amylase,?-glucosidase,?-glucosidase and polyphenol oxidase,increased significantly with the succession of BSCs.The enzyme activity of cellulase was the highest at the algae crusts stage.The enzyme activity of?-L-arabinosidase was the highest at the lichen crusts.The enzyme activity of polyphenol oxidase in the lower layer was higher than that in the BSCs layer,and the bare sand stage,algal crusts and moss crusts reached extremely significant levels(P<0.01).In the BSCs layer,soil enzymes related to nitrogen conversion,such as urease,protease,nitrite reductase and nitrate reductase,showed a similar trend to most of the soil enzymes related to carbon decomposition during the succession of BSCs,Which was significantly increased(P<0.05).The activities of urease,protease and nitrate reductase in BSCs layer were significantly higher than those in lower soil(P<0.05).The activities of urease and nitrate reductase in lower layer were also significantly increased with the succession of BSCs(P<0.05).The succession of BSCs can reduce the p H,increase soil water content,improve nutrients,such as the content of SOC,TN,NO3-,and NH4+,then they can improve soil environment.It provides carbon source and energy for heterotrophic microorganisms and promotes the growth of microorganisms such as bacteria and fungi.Different functional groups of microorganisms can secrete a variety of soil enzymes,such as sucrase,?-glucosidase,?-glucosidase,protease,urease and nitrate reductase.They degrade starch,cellulose and other organic compounds containing carbon and nitrogen.They also accelerate soil mineralization and promote nutrient accumulation.The development of BSCs is also beneficial to the formation of soil humus and the accumulation of soil nutrient pools.Microbial biomass carbon and nitrogen are active nutrient pools of soil,they can increase the activity of carbon and nitrogen related enzymes,improve the decomposition capacity of microorganisms,and increase the nutrient turnover rate.